The present invention generally relates to a method for attaching an optical fiber to a substrate, and more particularly to an optical fiber attachment method of improved efficiency designed to produce a highly stable optical fiber package.
New telecommunication methods, including the high-speed transmission of data, are now possible using laser technology. Specifically, the high speed transmission of information using laser light can be accomplished by the passage of such light through one or more optical fibers. Optical fibers typically used in data transmission are flexible and constructed of glass or plastic.
For optical fiber systems to function effectively, it is very important that the optical fiber or fibers be precisely aligned with the laser light source. Otherwise, the transmission of data may be impaired. Furthermore, once proper alignment is achieved, such alignment must be maintained during the assembly and operation of the system.
In typical optical fiber systems, laser light sources, including laser diodes, are mounted on support structures well known in the art, including supports of a pedestal-type configuration. As discussed above, optical fiber alignment with the laser light source is of prime importance. In order to accomplish this, conventional techniques involve securing the fiber directly to the support system associated with the laser. For example, one method first involves the application of a metal coating to an optical fiber. This coating typically consists of a primary nickel layer coated with an external layer of gold. The coated fiber is then positioned in front of the laser, and soldered to the laser support structure using lead/tin solder. However, numerous problems exist when this method is used. Primarily, the fiber is difficult to solder to the support structure and tends to alter its position during soldering. As a result, misalignment of the fiber with respect to the laser occurs.
Another conventional method involves placement of the optical fiber within a metal tube which is secured to the laser support structure. Again, alignment of the optical fiber using this system is very difficult, and affixation of the metal tube to the support structure frequently results in misalignment during the attachment process.
In order to minimize problems associated with the above-described methods, other procedures were tested in which the optical fiber was first joined to a substrate (typically manufactured of alumina), followed by attachment of the substrate directly to the laser support structure. However, when conventional methods were used to secure the optical fiber to the substrate, problems occurred. For example, one method involved soldering a gold-coated optical fiber to the substrate using lead/tin solder typically having a melting point of about 280.degree. C. The fiber was soldered to a metal pad on the substrate, such pad having an external layer of gold and at least one underlying metal binder layer. However, during soldering, the lead/tin solder formed an alloy with the gold on the optical fiber and the metals in the pad. As a result, a melting point depression occurred, lowering the melting point of the solder significantly below 280.degree. C. Thus, during attachment of the substrate to a laser support structure, the heat associated with such attachment frequently weakened the solder joint between the optical fiber and substrate, causing misalignment of the fiber with respect to the laser.
Another method used to secure the optical fiber to the substrate involved the use of "solder glasses." Solder glasses consist of low melting point glass compositions used to join materials together. However, joints made using solder glasses are very brittle, and are often weakened during affixation of the substrate to the laser support structure.
Finally, direct attachment of the optical fiber to a substrate is frequently characterized by problems involving internal stress within the completed assembly. Such problems result from a differential CTE (coefficient of thermal expansion) between the substrate and optical fiber. Coefficient of thermal expansion is a numerical value involving the change in size of a material per degree change in temperature. Stress-related problems most often result when an optical fiber is used having a substantially different CTE from that of its associated substrate. When heat is applied to such a system, both components will expand at different rates, thus resulting in possible bond damage or other physical harm to the optical fiber package.
Thus, a need exists for an optical fiber attachment method which enables the secure attachment of an optical fiber to a substrate while avoiding problems associated with joint weakness, internal stress, and optical fiber misalignment.